The invention relates to a method of projecting a mask pattern provided in a mask table on a substrate provided in a substrate table by means of a projection lens system and a projection beam, which method comprises the following steps:
providing a mask having mask reference marks in the mask table, PA0 providing a reference plate having reference plate marks in the projection beam, PA0 projecting the image of at least one mark of the mask on a corresponding mark of the reference plate by means of the projection beam and the projection lens system, PA0 detecting projection radiation originating from the illuminated reference plate mark by means of radiation-sensitive image detection detectors, PA0 electronically processing the detector signals to an alignment calibration signal and a magnification error signal, PA0 adjusting an alignment device and setting the magnification of the projection lens system by means of the last-mentioned respective signals, PA0 removing the reference plate from the projection beam and providing a production substrate in the projection beam, and PA0 repetitively projecting a production mask successively in different positions on the production substrate. PA0 the wavelength of the projection beam PA0 the gas pressure within the projection lens system PA0 the temperature within the projection lens system PA0 the composition of the medium in one or more of the compartments within the projection lens system PA0 the mutual distance between the lens elements in the projection lens system PA0 the zero-setting of the alignment device PA0 the zero-setting of the focusing device. PA0 the wavelength of the projection beam; PA0 the pressure within the projection lens holder; PA0 the mutual distances between the lens elements of the projection lens system; PA0 the composition of the medium in one or more of the compartments of the projection lens holder; PA0 the temperature within the projection lens holder PA0 the zero-setting of the alignment device; PA0 the zero-setting of the focusing device; PA0 the magnification of the projection lens system.
The invention particularly also relates to an apparatus for projecting a mask pattern on a substrate, which apparatus successively comprises an illumination system for supplying a projection beam, a mask table, a projection lens system and a substrate table and which further comprises an alignment device and a magnification setting device used for aligning the mask and the substrate with respect to each other during the production projection process and for setting the magnification with which the mask pattern is imaged on the substrate, and an image detection device used for checking, prior to a production projection process, a mask image formed by means of the projection lens system and the projection beam, said image detection device comprising a reference plate having at least one mark on which the image of a corresponding mark of the mask is formed and a radiation-sensitive detection system for converting projection radiation originating from at least one reference plate mark into electric signals.
Such a method and device are described in U.S. Pat. No. 4,504,277 which relates to an apparatus for repetitive and reduced imaging of a mask pattern, for example, the pattern of an integrated circuit (IC) on one and the same substrate, while the mask pattern and the substrate are moved with respect to each other between two successive illuminations, for example, along two mutually perpendicular directions in a plane parallel to the substrate plane and the mask plane.
Integrated circuits are manufactured by means of diffusion and masking techniques. A number of production masks with different mask patterns are consecutively projected on one and the same location on a semiconductor substrate (production substrate). Between the consecutive projections on the same locations the production substrate must undergo the desired physical and chemical variations. To this end the substrate must be removed from the apparatus after it has been illuminated with a production mask pattern, and after it has undergone the desired process steps, it must be placed in the apparatus again in the same position so as to illuminate it with a second production mask pattern, and so forth, while it must be ensured that the images of the second production mask pattern and the subsequent production mask patterns are positioned accurately with respect to the substrate.
Diffusion and masking techniques can also be used in the manufacture of other structures having detailed dimensions of the order of micrometers, for example, structures of integrated optical systems or guiding and detection patterns of magnetic domain memories and structures of liquid crystal display panels. In the manufacture of these structures the images of mask patterns must also be aligned very accurately with respect to a substrate.
In connection with the large number of electronic components per unit of surface area of the substrate and the resultant small dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which the integrated circuits are manufactured. The position where the consecutive production masks are imaged on the production substrate must therefore be established more and more accurately. Due to the smaller depth of focus of the projection lens with which smaller details can be projected, it must also be possible to focus more accurately.
In order to be able to realise the desired, very precise positioning accuracy within several tenths of one micrometer of the image of the mask pattern with respect to the production substrate, the projection apparatus comprises a device for aligning the production substrate with respect to the production mask pattern. With this device an alignment mark provided in the production substrate is imaged on an alignment mark provided in the production mask. If the image of the substrate alignment mark accurately coincides with the mask alignment mark, the production substrate is correctly aligned with respect to the production mask pattern. The main element for imaging the production substrate mark on the production mask mark is constituted by the projection lens system with which the production mask pattern is imaged on the production substrate.
This projection lens system is designed and optimally corrected for the wavelength of the projection beam. This wavelength is as small as possible so that the smallest possible details can be projected at the same numerical aperture of the projection lens system. In the current projection apparatuses this wavelength is, for example, 365 nm with which line widths of approximately 0.7 .mu.m can be projected. The alignment beam, i.e. the beam used in the alignment device has such a wavelength that the photoresist on the production substrate is insensitive thereto so that such a beam cannot cause a change in the photoresist provided on the substrate and is not weakened by this photoresist. This alignment beam is, for example, a Helium-Neon laser beam with a wavelength of 633 nm. Although the wavelength of the alignment beam is not adapted to the projection lens system, the alignment marks of the production mask and the production substrate can be aligned satisfactorily with respect to each other if a correction element, for example, a lens is arranged in the path of the alignment beam only.
However, since the projection beam and the alignment beam have different wavelengths, the problem remains that changes of, for example, ambient parameters such as temperature have a different effect on the images which are formed with the projection beam and the alignment beam, respectively. Consequently, the alignment device may detect a satisfactory mutual alignment of the alignment marks associated with this device, while the mask image formed by means of the projection beam is incorrectly located with respect to the substrate. A mechanical drift in the projection apparatus which cannot be detected with the alignment device may also occur. It is therefore necessary to calibrate the conventional alignment system periodically, for example, once or several times a day.
To this end the apparatus according to U.S. Pat. No. 4,540,277 comprises an image detection device for checking, inter alia, the image formed by the projection beam. This device comprises a reference plate which is fixedly connected to the substrate table and in which four radiation-transmitting slits are provided and it is further provided with four radiation-sensitive detectors arranged under the slits and provided in the substrate table. For the periodical inspection of the projection apparatus a test or reference mask is provided on the mask table, which mask has marks which correspond to the marks on the reference plate. This plate is slid under the reference mask and in the path of the projection beam whereafter this beam is switched on, forming images of the four reference mask marks on the four marks of the reference plate. The four detectors which receive the radiation transmitted by the reference plate marks then supply signals from which the extent of alignment of the reference mask with respect to the reference plate can be derived. Since the reference plate is also provided with alignment marks cooperating with the conventional alignment system, it can be ascertained whether an extent of alignment detected by means of the image detection system corresponds or does not correspond to the extent of alignment as is measured with the conventional alignment device. With the help of this checking, the last-mentioned device can be calibrated.
U.S. Pat. No. 4,540,277 also notes that a magnification error of the image formed with the projection beam can also be measured by means of the image detection device by establishing whether the images of the mask reference marks cover the marks of the reference plate to an equal extent. Since the projection lens system in the apparatus according to U.S. Pat. No. 4,540,277 is non-telecentric at the object side, or mask side, a magnification error can be eliminated by adapting the distance between the mask and the projection lens system.
The known image detection device operates in transmission so that it is necessary to provide the detectors in the substrate table. These detectors require extra space so that the substrate table must be bigger and heavier and must also be moved over larger distances during the measurements. This results in additional problems for the servo devices and has a negative influence on the measuring and setting accuracies.
The marks of the known image detection device consist of slits which may be two-dimensional so as to enable measurement in two mutually perpendicular (X-Y) directions. To achieve the desired positioning accuracy, the slits must be very narrow and consequently the quantity of radiation on the detectors is small and the signal-to-noise ratio leaves much to be desired, while very strict requirements must be imposed on the geometry of the slits.
Since only one detector is present for each mark of the reference plate, the X and Y positions for each mark of the reference plate with respect to the associated reference grating mark cannot be determined separately even when using two-dimensional slits.
The above-mentioned problem that, due to the different wavelengths of the alignment beam and the projection beam, an alignment detected by the alignment beam as being correct does not need to correspond to a correct alignment in projection radiation will be greater as more electonic components per unit of surface area are to be provided on the substrate. These components must then have even smaller dimensions and a projection apparatus is required which can make images in a repetitive manner whose details or line widths are considerably smaller than one micrometer. This means that the resolving power of the projection lens system must be increased. As is known, this resolving power is proportional to NA/.lambda. in which NA is the numerical aperture of the projection lens system and .lambda. is the wavelength of the projection beam. The numerical aperture is already fairly high, for example, NA=0.48 for known projection lens systems.
Another important factor is that the depth of focus of the lens system, which should be as large as possible, is proportional to .lambda./NA.sup.2 so that an enlargement of the numerical aperture is more detrimental to the depth of focus than a reduction of the wavelength.
Substantially the only possibility which is then left for realising the desired imaging with details of the order of 0.4 .mu.m with the desired depth of focus is to make use of a projection beam having a considerably smaller wavelength than has hitherto been conventional. In order to be able to project the mask pattern on the substrate by means of such a short-wave beam, lens elements of quartz must be used. Since quartz is very dispersive, the radiation used should have a very narrow wavelength band. Therefore, a radiation source will have to be used which emits a large power within a narrow wavelength band. A real possibility then is the use of an excimer laser, for example, a Krypton-Fluoride laser having a wavelength of 248 nm, an Argon-Fluoride laser having a wavelength of 193 nm, or an Nd-YAG laser whose frequency is quadrupled and which has a wavelength of 256 nm. A projection beam must then be used whose wavelength is of the order of 2.5-3.2 times the wavelength of the alignment beam.
When forming images with said small dimensions of the details, not only the problems mentioned in U.S. Pat. No. 4,540,277, namely the alignment, image rotation, magnification error and anamorphotic imaging error will be greater but new problems will also occur which predominantly relate to the image quality of the projection lens system.
Although there has been a breakthrough in the field of projection lens systems, enabling projection line widths of the order of -0.4 .mu.m in an image field of the order of 25 mm, these projection lens systems are very sensitive to variations of the ambient parameters such as air pressure and temperature. Due to the high dispersion of the lens material, a change of the wavelength of the projection beam influences the imaging quality, i.e. the position and the quality of the image formed with this beam. Problems with third-order distortion, image astigmatism and image field curvature may arise in the projection apparatus. The novel generation of projection lens systems with their very high resolving power and a relatively large image field have a very small depth of focus so that focusing errors, inter alia resulting from the larger wavelength dependence of the projection lens system, have an increasing influence. These errors should be detected very accurately and it will then be necessary to calibrate also a focus error detection device periodically. Moreover, the influence of mechanical drift will increase as the image details become smaller.
The article "Characterization and Set-up Techniques for a 5.times. Stepper", by T. A. Brunner in SPIE, Vol. 633: Optical Microlithography V 1986, pp. 106-112, describes a so-called scanning slit image detector which comprises a reference plate on which a plurality of fluorescent strips is provided, which reference plate must be provided on the substrate table of a projection apparatus so as to scan the image of a test mask. The diffuse scattered radiation emitted by the fluorescent strips is received by one detector. In the known image detection device radiation from the entire pupil of the projection lens system can reach the relatively large detector so that the signal-to-noise ratio of the measuring signals leaves much to be desired. Moreover, there is a risk that non-uniform scattering takes place and since only one detector is used, the effective measurement, via the image detection device, in the pupil of the projection lens system may be asymmetrical. In the known image detection device an incoherent detection method is used.